This invention relates to metal oxide varistors. More specifically, this invention relates to varistors which comprise a composite of finely ground metal oxide varistor ceramic in a plastic resin matrix.
There are a few known materials which exhibit non-linear resistance characteristics and which require resort to the following equation to relate current and voltage quantatively. EQU I = (V/C).sup..alpha.
where V is a voltage between two points separated by a body of the material under consideration, I is the current flowing between the two points, C is a constant, and .alpha. is an exponent greater than 1. Materials such as silicon carbide exhibit non-linear or exponential resistance characteristics and have been utilized in commercial silicon carbide varistors, however, such non-metallic varistors generally exhibit an .alpha. exponent of not more than six.
Recently, a family of polycrystalline metal oxide varistor materials have been produced which exhibit an .alpha. exponent in excess of ten. These new varistor materials comprise a sintered body of zinc oxide crystal grains, including additionally an intergranular phase of other metal oxides and/or halides, for example: beryllium oxide, bismuth oxide, bismuth fluoride, or cobalt fluoride, and are described, for example, in U.S. Pat. No. 3,682,841 issued to Matsuoka et al and U.S. Pat. No. 3,687,871 to Masuyama et al.
The non-linear resistance relationship of metal oxide varistors is such that the resistance is very high (up to at least 10,000 megohms) at current levels in the microampere range, and progresses in a non-linear manner to an extremely low value (tenths of an ohm) at high current levels. The non-linear resistance characteristics result in a voltage versus current characteristic wherein the voltage is effectively limited, the voltage limiting or clamping action being more enhanced at the higher values of the .alpha. exponent. Thus, the voltage versus current characteristics of metal oxide varistor material is similar to that of the Zener diode with the added characteristic of being symmetrically bidirectional. The "breakdown voltage" of a metal oxide varistor device is determined by the particular composition of the material and by the distance between the electrodes on the varistor body.
Metal oxide varistors of the prior art are fabricated by pressing and sintering a mixture of metal oxide powder at temperatures in the region of 1300.degree. C. to form a generally hard, brittle ceramic body. Circuit components of metal oxide varistor ceramics are generally formed by pressing and sintering disks of the material, applying the electrodes, for example, by painting or screening conductive materials on the surface of the disks, affixing wire leads, and encapsulating the finished component in a suitable dielectric.
It has been suggested that metal oxide varistor ceramics be pressed or machined into complex shapes and bonded to metal terminals and contacts to form specialized circuit components, as for example in U.S. Pat. Nos. 3,742,420 to Harnden and 3,693,053 to Anderson. The manufacture of metal oxide varistors in shapes other than flat disks requires dimensional control, however, which is difficult to attain in a sintering process (due to shrinkage and deformation) and the temperatures encountered in the sintering processes are generally incompatible with common, low cost electrical metals. Machining of sintered parts generally involves grinding brittle materials and is not an economically attractive process for large scale mass production.
Metal oxide varistor components have been formed in the prior art by screening a paste of ground metal oxide varistor ceramic and glass frit on a dielectric substrate and firing to produce a thick film device; as described for example in U.S. Pat. No. 3,725,836 to Wada.